My goal was to add some type of blinker style, turn signal lights to my bike. The lights had to be removable and composed of bits & pieces from spare parts laying around the house. I'm hoping this servers more as inspiration than a step-by-step how to, your scrap pile of recycled glory may be different from mine.

Parts I used include:

Three (3) red blinky lights
You're going to need three independently controlled lights, one for each side and one for center. The center light is important. The left and right lights are much closer together than on a car and telling one direction from another without a center reference is no easy.

Two (2) toggle switches

A six (6) foot USB extension cable

Some scrap wire, zip ties, and other tidbits

Step 1: Lights

The lights you choose should be as simple as possible. Some bike lights can get rather complex with their multiple flasher functions and dozens of LEDs. The lights I found have three LEDs and a solid-strobe-off function button. The center light I'm using if the safety light already on my bike, the two others were cheap clip-on running lights.

We'll be leaving the center light alone for now. We want on/off control of each side light via the toggle switch we will be attaching shortly.
With the light on, open it up and poke around a bit until you find two contacts that turn the light off, there may be a pair for each LED or one pair that extinguishes them all, To each of these points attach a length of scrap wire and test. With the light turned on, you should be able to touch the wires together to turn it on and off.
Do this with both side lights and run the wires out a small hole in the lights casing.

Step 2: Light Assembly

We are going to use the USB cord to connect the switches at the front of the bike to the lights in the back.
First we need to combine the three lights into one. I am using an existing bike light as my center, this light is removable and has a permanent mount already bolted to my bike.
To attach the two side lights I added a few inches of an old paint stick, a couple screws, staples, and a generous amount of hot glue.
Cut your USB about four inches from one end (doesn't matter which), Strip the wires back a little way and you should find four separate wires inside (you may have more, only need four). With each piece of USB pair up two wires for left and two for right (example: red & green = left, black & white = right).
Now with your short piece of USB, attach the left and right lights to each pair of wires. Secure the USB cord to your light assembly and top off with a little more glue to make sure nothing's going to vibrate loose.

Step 3: Switches

Attach the two toggle switches to the other USB segment. I salvaged these from an old toy keyboard. Keep the wire pairs the same from earlier; left pair to one toggle, right pair to the other.

You should now be able to test the full turn signal assembly.
Turn on all three lights, if they have multiple functions I suggest turning the center light solid with the sides blinking.
Connecting the male-female ends of the USB should give you on/off control of the two side lights, via the toggle switches; one on/off for left, one on/off for right.

Attaching all this to your bike is going to depend on the parts you used and the style of bike you own.
Six feet of USB should be more than enough to reach from handlebars to the rear of the bike, making sure to leave slack for twisting and turning.
The light used the same mount from earlier, when combining lights be sure not to get in the way of the bits necessary to attach and remove this center light.
The switches I was able to wedge into a corner of the brake grips, held in place with a couple zip-ties and electrical tape.
The USB can be attached to the bikes frame with more zip-ties, leave a few inches of slack between the handle bars and the frame, and be careful not to put any tension on your brake cables.

Step 4: Ride Safe

Just a reminder to ride safe out there.
Keep in mind that lights like these are almost worthless in the day light, so remember your hand signals.
And stay aware of your surroundings, nothing substitutes a little education and caution.

(great bicycle safety information site http://bicyclesafe.com/ )

~james

...this is my first instructable, constructive comments are always welcome =) .....